Mad Max director George Miller was "heartbroken" over Mel Gibson's string of scandals in recent years, and said plans for a new trilogy prompted him to pick a younger actor for revisiting the post-apocalyptic auto action franchise.

Miller had originally hoped to bring Mel back for a fourth film in the role of Max Rockatansky. But a series of delays led to recasting, with Tom Hardy, 36, stepping into the road warrior's black boots.

"I have a great affection for Mel. I was really heartbroken to see him go into that," Miller said in a recent interview. "But it's a new time. I hope Mel gets to act in more movies because I think he's a wonderful actor. But I think he's an amazingly good director."

Miller describes Mad Max: Fury Road as an extended chase taking place over three days. The film, due out next summer and also starring Charlize Theron, features minimal dialogue. The screenplay consisted of storyboards - sketches of each planned shot - rather than a conventional script.

"You're picking up the characters and the backstory as you go," Miller said. "And in order to create that backstory, we found ourselves having written two other screenplays. One of them is completed. The other one is in the form of a kind of unedited novel. So by the time we got there, we realised we've got a couple more Mad Max stories to tell, and that ... required us to cast someone who was younger."

Miller showed footage from Mad Max: Fury Road at the recent Comic-Con International in San Diego. He is still finishing work on the film, his return to live action after directing the animated Happy Feet in 2006 and 2011's sequel.